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Can we create star topology on Canopen network? Are there any rules?

Published date: 21 May 2019

Star topology on canopen is not possible. Canopen allows only bus topology, though certain tolerance to sub lines exists, depending on bus speed.
Stub lines (also called drop cables or branches) are those parts of wiring that exist between a node and nearest bus junction. For example if you use CANTAP boxes (VW3CANTAP2 or TSXCANTDM4), the stub lines are branches between CANTAP and ATV. Attached picture with topology shows in yellow all stub lines.
The goal is to have total length of all branches as minimum as possible, because the branches causes signal reflection and distort signal quality on the bus.
The rules for max tolerated branches length, for single branch limit and for total length of branches connected to the bus are shown in picture.
For example, at required bus speed 500kb/s, the max length of single stub is 5m. But total length of stub lines per one tap is 10m. It means if you use TSXCANTDM4 and 4 ATVs connected to it, the average length between one ATV and CANTAP can be max 2.5m
There is also one more criteria, that if you use more CANTAP boxes, they should not be closed to each other as distance between taps must be at least 60% of total stub lines per tap.

Example: In case of 4xATVs with stub lines (1m, 1m, 3m, 0,3m) connected to TAP1 and another 4 ATVs with stub lines (3m, 3m, 1m, 1m) connected to TAP2, the distance between taps TAP1 and TAP2 must be > 60% (3+3+1+1) = 4.8m
CANTAP boxes are not entirely necessary for creating bus topology. The bus network can be created directly in the canopen conector (SUBD-9) or if you use open style canopen card (VW3A3628), then bus is created directly on card terminals. Also Canopen daisy chain card with 2xRJ45 connector is OK, as canopen signals between RJ45 connectors are hardwired. So if you remove VW3A3608 card from ATV, the bus will not be interrupted.

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